VSE Fade audio in and out
In the View menu, switch on Waveforms and F-curves.
Select the strip.
In Add menu, select Fade - Fade In Out
(Alternatively right click in the Sequencer area and select the fade options there - here is also an option to Clear Fades)

Gif:

If you want to animate with the diamond, first set the value then click the diamond, so it is filled(you set a key-frame). Having the f-curves visible makes it much more visible what happens. Alternatively you could change ex. the File Browser area to Graph Editor: